Tyndall National Institute - Doctoral Theses
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Item Towards a GaN-on-sapphire photonic integrated circuit via micro-transfer printing(University College Cork, 2024) O'Brien, Megan; Corbett, Brian; Peters, Frank H.; Science Foundation IrelandPhotonic integrated circuits (PICs) can alleviate the pressure on existing infrastructure for increasing bandwidth requirements by potentially offering faster data transmission with low energy consumption all on the same chip. The ability to integrate active lasing sources to waveguide platforms while also maintaining low-loss coupling is key for efficient PICs.This research focuses on the development of an AlGaN/GaN-on-sapphire platform and the integration of 1.3 µm edge-emitting InP-based multi-quantum well (MQW) and GaAs-based quantum dot (QD) lasing devices to the platform via micro transfer printing (µTP). Gallium nitride (GaN) can be used alternatively to traditional Si-based photonics due to its broad spectral transmission, birefringence, refractive index suitable for fiber coupling, and the possession of both the linear electro-optic effect and non-linear properties. First, we design an AlGaN/GaN structure that creates a single-mode, broad passive spot size for relaxed alignment tolerance of the transfer-printable laser-to-waveguide. Coupling losses as low as 0.6 dB were calculated for the optimal positioning of the laser. We also outline a double-tapered mode adapter that converts the dimensions to more suitable structures for light-routing. The integration of LiNbO3 coupon to the AlGaN/GaN platform is explored through simulations in order to enhance the electro-optic property, and a Mach-Zehnder modulator (MZM) design is proposed. Additionally, the simulations of (i) directional couplers, (ii) polarization converters and (iii) grating couplers on an AlGaN/GaN platform are presented in this work. Next, the fabrication of AlGaN/GaN waveguides devices through BCl3-based etching recipes, along with the challenges of achieving smooth and vertical waveguide sidewalls, are discussed. A BCl3 dry etch for forming a trench into sapphire is also developed for µTP alignment purposes. The AlGaN/GaN waveguides were then characterized, and we distinguish the differences in propagation losses between the TE and TM polarizations.The average propagation losses ranged between 2.4-14.7 dB/cm for both polarizations. Following on from this, both the InP-based MQW and the GaAs-based QD laser coupons were characterized and transfer-printed and coupled to the fabricated GaN waveguides. A coupling output of 2.3 mW at 80 mA was achieved for the QD laser, giving a coupling efficiency of 10.3 %. The results show promise for GaN to be a suitable platform for integrated photonics.Item Micro cameras as adjunct tools in biomedical applications(University College Cork, 2024) Niemitz, Lorenzo; Andersson-Engels, Stefan; Burke, Ray; Sorensen, Simon Toft; Science Foundation IrelandBiomedical imaging is one of the most impactful periprocedural tools and is used across a wide range medical applications. The eyes of the clinician are a key component in surgical decision making, and an image is often the first step in both diagnostic and interventional procedures. With the miniaturisation of CMOS image sensors, micro-cameras are being investigated for use in biomedical imaging and the development of biomedically specific sensors has begun. This thesis makes several important contributions to the use of these micro-cameras as adjunct tools that meet a number of unmet clinical needs in a variety of interventional procedures. To achieve this integration capabilities are demonstrated and a micro-camera based technology platform is developed. The micro-camera is integrated at the tip of various devices, along with multi-spectral fibre illumination, automated readout, control, and image processing; into a compact format suitable for clinical use. The technology is then applied to a number of applications, with the system tailored to the specific use cases. The verification of procedural success in a cardiovascular intervention is investigated. A micro-camera based 2.30 mm catheter device is developed and tested on an animal model. It is shown to be able to image in the challenging environment of the blood field. Here the procedure can be recorded and we are able to extract physiological information in a step toward verification of device placement as well as success of ablation procedures in the heart. The dimensions are pushed to 1.19 mm to navigate into the peripheral lung. This achieves imaging deeper than with a traditional bronchoscope, while maintaining the ability to provide multi-spectral illumination via fibre optics. This is demonstrated on ex-vivo tissue. The micro-camera platform makes an impact on breast conserving surgery procedures, where for the first time the periprocedural imaging of micro-calcifications is investigated. This enables the intra-surgical detection of micro-calcifications up to 2.00 mm in the resection margin. To do this a multi-spectral diffuse optical imaging technique for surgical guidance is proposed, and used to detect micro-calcifications. A rigorous co-registration method is developed to validate the data, and image processing techniques proposed to aid the automated detection. Finally, polarisation resolved imaging is investigated using the microcamera. Here the platform offers a solution for flexible polarisation resolved imaging systems. An initial prototype of both a miniature and benchtop system is presented along with images on optical elements and bulk tissue samples. Line of sight to tissue imaging, and to investigate multi-spectral polarisation resolved imaging is discussed.Item Micro-transfer print integration of high-speed photodetectors to SOI platform(University College Cork, 2024) Muthuganesan, Hemalatha; Corbett, Brian; Peters, Frank H.; Science Foundation IrelandPhotonic integrated circuits (PICs) offer a pivotal solution to meet the escalating demands of data bandwidth and used in data centres, LIDAR and optical sensing. Among various approaches, micro-transfer printing emerges as a rapid and universally compatible integration technology to generate PICs. To implement this technology, it is necessary to separate the devices from their original substrate using a release layer and transfer print them to designated location on target wafer. This enables the heterogeneous integration of numerous active devices from multiple wafers onto a single wafer, resulting in a compact and highly functional PIC. In addition to being a room temperature process, it is also cost-effective allowing reuse of expensive III-V substrates. This thesis demonstrates the application of micro-transfer printing technology in integrating ultra-thin, high-speed InGaAs photodetectors with silicon waveguides through various coupling mechanisms. The thesis begins with optimizing the optical power coupled between the waveguide and the photodetector (PD), as even a small gap between them leads to optical mode loss at the interface. A co-planar in-fill idea is experimented by transfer printing the PD’s absorber region at the same height as silicon waveguide. To fill the gap, evaporated amorphous silicon (a-Si) with a refractive index of 3.1 in the telecom wavelength range is utilized. This strategy aims to achieve maximum coupling. In transfer printing process, it is crucial to efficiently release the PD coupons along with smooth interface, for successful printing on target wafer. This work introduces for the first time, a combination of InGaAs and AlInAs as release layers for InP based devices yielding double the etch rate and almost isotropic etch compared to individual release layers (InGaAs or AlInAs). This facilitates direct bonding of the PD coupons to the target wafer since the interface roughness is extremely low as 0.2 nm over an area of 10 µm x 10 µm, contributing to high-speed of the PD. An excellent selectivity of 5410 is obtained with InP, which is 3.4 times higher than AlInAs and 7 times higher than InGaAs as release layers. These outcomes are accomplished at room temperature thus saving time, cost, and energy for the entire process. The above optimized processes are used in integration of high-speed InGaAs photodetector to SOI platform, by direct bonding, with 100 % yield. An ultra-thin 675 nm epitaxial stack is grown with dual release layer on InP substrate, where a PD of size 21 µm x 57 µm is fabricated. Such small dimension has the potential to fit 1 million devices on a 75 mm InP wafer. The same photodetectors are coupled to silicon waveguides via evanescent, grating and butt coupling mechanisms on the same target wafer. These PDs exhibit a maximum responsivity of 0.6 A/W, 47 nA dark current and with a data communication rate of 50 Gbps with on-off keying. In future, this unique integration approach can be universally applied to any III-V active device like lasers and modulators to realise compact and high performance photonic integrated circuits.Item Multiferroic investigations of Aurivillius phase thin films(University College Cork, 2023) Colfer, Louise; Keeney, Lynette; Long, Brenda; Royal Society; Science Foundation IrelandIn recent years, the amount of data being created and processed is growing at a much faster rate than the rate of computational storage technology development. With CMOS technologies reaching their miniaturisation limits, new disruptive materials are needed to increase data storage capabilities. Technological road-maps have identified room temperature, non-volatile magnetoelectric multiferroic materials as promising candidates for memory scaling within future memory storage devices. Although multiferroic memory devices have the potential to revolutionise memory storage technologies, commercial devices successfully utilising multiferroics have not yet come to fruition. The focus of this thesis is to understand and optimise a rare example of a room temperature magnetoelectric multiferroic, Bi6TixFeyMnzO18 (B6TFMO; x = 2.80 to 3.04; Y = 1.32 to 1.52; Z = 0.54 to 0.64). Aurivillius phase materials, (Bi2O2)(An−1BnO3n+1), where ferroelectric perovskite units are interleaved between dielectric [Bi2O2]2+ layers, are flexible scaffolds for technological applications. While earlier studies indicated that B6TFMO is a promising material for future memory devices, my thesis presents significant advances in the characterisation, understanding and optimisation required towards implementing the material in fully realised devices. In this work, correlation between the octahedral tilting and atomic-level structural distortions with functional electronic and magnetic properties of B6TFMO were determined, revealing that crystal field splitting of the Ti4+ octahedra is influenced by its position within the Aurivillius unit cell. Theoretical calculations determined that this is predominantly driven by changes in the extent of tetragonal distortion along the c-direction. Atomic scale mapping of polar displacements reveals this has a direct impact on the ferroelectric properties. Polarisation is largest towards the outer perovskite cells, correlating with an increased extent of local tetragonal distortion of octahedral geometries. Experiments demonstrate that tilting of the BO6 octahedra competes with the extent of tetragonal distortion of the TiO6 octahedra, where the degree of octahedral tilting increases towards the central layers of this Aurivillius system, where the magnetic cations preferentially partition. This work presents the first indication that octahedral tilting might be an important enabler of long-range magnetic interactions and subsequent multiferroic behaviour in B6TFMO. Delving deeper into fundamental understandings of B6TFMO’s antipolar and magnetic behaviour, the purposeful inclusion of structural defects within the layered structure of B6TFMO and how they can impart elastic strain and electrostatic energy changes which in turn influence polar behaviour is explored. The findings show that the vicinal sapphire substrates (mis-cut angle 0.2 o to 10 o) are successful for promoting the propagation of sub-unit-cell defects and disruptions to the periodicity of the Aurivillius phases. This has a marked effect on the film morphology and ferroelectric properties. Macroscopic and local measurements show that defect, crystal grain and ferroelectric domain density increases with increasing substrate mis-cut angle. Atomic resolution polarisation mapping showed that charged domain walls alongside exotic polar vortices are facilitated by OPBs when two OPB defects are spaced 5 nm apart. This work provides insight into methods for successfully controlling defect levels and how polar vortex domain walls and charged domain walls are promoted within layered multiferroics by tailoring the underlying substrate that the film is grown on. Moving on from vicinal sapphire surfaces, patterned sapphire with 3D domes were used to encourage the growth of the Aurivillius grains towards an upright geometry. An increased number of non-(00l) reflections were present in the B6TFMO films on patterned sapphire along with evidence from STEM imaging showing that B6TFMO grains grow along the incline of the patterned sapphire domes. With the growth of the crystal grains towards an upright geometry it would be expected that access to the major a-axis polarisation via out-of-plane measurement would be improved, however with a maximum inclination angle of 60 ° achieved with the 3D dome architectures, the out-of-plane piezoresponse of the samples remained weaker than the in-plane piezoresponse. Studies of the magnetic properties of the films demonstrated that the B6TFMO samples were ferromagnetic at room temperature. These findings provide further evidence of room temperature multiferroic behaviour in B6TFMO. Lastly, the role of bismuth excess and substrate strain were investigated to optimise the epitaxial growth of B6TFMO via DLI-CVD. A single-step deposition method on epitaxial substrates was developed to allow the successful synthesis of continuous 45 nm thick B6TFMO films at thicknesses relevant to applications as piezoelectric actuators, sensors and energy harvesters. These films nucleated via a layer-by-layer growth mode and were found to have a strong in-plane ferroelectric response with isotropic domains. Film purity was enhanced with utilisation of epitaxial substrate with appropriate lattice match to B6TFMO and by optimising the amount of bismuth precursor used. In this work, progress was made towards the optimisation of epitaxially grown B6TFMO films, allowing greater control of film orientation and augmenting strain-induced enhancement of multiferroic properties in future data storage devices. Overall, this research has increased understanding of the fundamental mechanisms governing the ferroelectric and ferromagnetic properties of B6TFMO. The work has elucidated some of the key requirements fundamental to the manifestation of polar topologies and has created strategies for the tailoring of novel polar topologies. This combination of new material understanding and new growth optimisation of room temperature multiferroics contributes to solving the ‘big data’ problem. Application of B6TFMO in future technologies based on ultra-high density, energy efficient memory devices, spintronic devices, multilevel resistance control (memristive and synaptic devices) and energy-efficient neuromorphic “brain inspired” devices are envisioned.Item Toward single-growth monolithically integrated electro-absorption modulated lasers(University College Cork, 2023) Mulcahy, Jack; Peters, Frank H.; Corbett, Brian; Science Foundation Ireland; Rockley PhotonicsEvery year the demand for bandwidth is growing exponentially due to the emergence of data-intensive services such as high-definition video streaming, cloud-based computing, and machine-to-machine communication. This rapid expansion is primarily driven by the extensive deployment of fibre-based optical communication networks. Consequently, there is an increasing need for photonic components to meet the requirements of these networks, which are expanding both in geographical coverage and terminal density. To satisfy this demand, the photonics industry must enhance its production capabilities and adopt more efficient fabrication processes. A crucial aspect of streamlining fabrication involves eliminating slow and costly processes. In photonics fabrication, epitaxial regrowth and advanced lithography steps are typically time-consuming and expensive, making them prime targets for process optimisation. Moreover, the integrated electronics approach provides valuable insights by enabling the monolithic integration of multiple photonic components fabricated simultaneously. This integration technique allows for the creation of highly complex circuits while reducing overall fabrication complexity. This research focuses on a key component at the heart of photonic circuits: the tunable single-mode laser. The aim is to contribute to the development of components that can be fabricated without the need for regrowth or advanced lithography. Additionally, the study emphasises the importance of monolithic integration, specifically with electro-absorption modulators (EAMs). By integrating EAMs with tunable lasers, the resulting devices can offer enhanced functionality and performance, leading to more efficient and compact photonic systems. The issue at hand, however is the varied epitaxial requirements of lasers and EAMs, which provides a noted barrier to a monolithic, regrowth-free integration process. This thesis aims to advance the development of single-growth monolithically integrated externally modulated lasers (EMLs) based on electro-absorption modulators (EAMs). The design of quantum well structures is explored, revealing the significance of introducing an imbalance in the position of the quantum wells to optimise the transit times of carriers in EAMs, thus maximising the bandwidth. Simulation studies on epitaxial structures led to the identification of an optimal material that balances the performance of lasers and EAMs, providing an ideal platform for EML fabrication. Different laser designs are investigated, including slotted Fabry P\'erot lasers and snails, with a focus on achieving a redshifted single-mode laser. Simulation models are developed to predict laser reflectivity and spectral output, which were verified through fabrication and testing. The optimal laser design for integrated EMLs was determined through critical evaluation with a laser being produced with $>$ \SI{40}{dB} SMSR and a tuning range of \SI{60}{nm}. A high-speed process for fabricating EAMs is developed, featuring optimised lithographic mask layers for the isolation of contact pads and metal bridges to reduce parasitic capacitance. The resulting EAMs exhibited a predicted bandwidth of approximately \SI{80}{GHz}. Drawing upon the knowledge gained from laser and EAM simulation, fabrication, and characterisation, a new high-speed process for EMLs is devised. The o-band lasers and EAMs were designed based on optimal principles determined in previous chapters. The fabricated single-mode lasers were successfully matched to simulated models. Further analysis identified potential avenues for improving future EML fabrication yields. In summary, this thesis provides valuable insights and tools for the creation of single-growth monolithically integrated electro-absorption modulated lasers. The journey spans from material design to device outputs, with the aim of enabling readers to replicate and enhance the development of EMLs.